Urticaria pigmentosa (UP) is a relatively common disorder in pediatric patients, and little is known regarding the somatic and germline genetic variants associated with the disease. The University of Minnesota Masonic Children's Hospital is a regional referral center for pediatric patients with mast cell disorders. Collaborators on this study include several University departments including: Pediatric Dermatology, Pediatric Oncology, the Biomedical Genomics program, Lab Medicine and Pathology department. We hypothesize that because of differences observed in the clinical behavior of pediatric- and adult-onset mast cell disease, specifically UP, we will identify novel somatic gene variants in addition to c-KIT . We further hypothesize that we will observe novel germline genetic variants in pediatric UP distinct from what has previously been described in adults.